setMinMergeDocs

public void setMinMergeDocs(int minMergeDocs)

Sets the minimum size for the lowest level segments.
Any segments below this size are considered to be on
the same level (even if they vary drastically in size)
and will be merged whenever there are mergeFactor of
them. This effectively truncates the "long tail" of
small segments that would otherwise be created into a
single level. If you set this too large, it could
greatly increase the merging cost during indexing (if
you flush many small segments).